Radio frequency identification (RFID) technology is becoming increasingly important for logistics concerns, material handling and inventory management in retail stores, warehouses, distribution centers, buildings, and like controlled or confined areas. An RFID system typically includes at least one RFID reader, also known as an RFID interrogator, and an RFID tag that is usually attached to, or associated with, an individual item, or to a package for the item. The RFID reader interrogates or scans one or more RFID tags in its coverage range by transmitting a radio frequency (RF) signal, and the RFID tag, which senses the interrogating RF signal, responds by transmitting a return RF signal. The RFID tag either generates the return RF signal originally, or reflects back a portion of the interrogating RF signal in a process known as backscatter. The return RF signal may further encode data stored internally in the tag, such as a number. The return signal is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), or any combination of attributes, and so on.
The RFID tag typically includes an antenna, a power management section, a radio section, and frequently a logic section, a memory, or both. In earlier RFID tags, the power management section included an energy storage device, such as a battery. An RFID tag with an active transmitter is known as an active tag. An RFID tag with a passive transmitter is known as a passive tag and backscatters. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered solely by the RF signal it receives. An RFID tag that backscatters and is powered by an on-board battery is known as a semi-passive tag.
The RFID system is often used in an inventory monitoring and tracking application. For example, in order to take inventory of RFID-tagged items in a retail store, it is known to position at least one RFID reader overhead in a controlled area or inventory location, and then, to allow each overhead reader to automatically read whatever tagged items are in the coverage range of each reader. For superior RF coverage, it is known to provide each overhead reader with an array of antennas, such as a phased array antenna, that generates a multitude of scan beams that are steered both in azimuth, over an angle of 360 degrees, and in elevation, over an angle of about 180 degrees.
As advantageous as such known automatic inventory-taking RFID systems utilizing phased array antennas have been, it has proven difficult to optimize reader performance, because there are hundreds of scan beams for each reader, and each scan beam is activated individually and in a fixed sequence. Because only a single scan beam is active at a time, scanning though multiple individual scan beams is likely to be required to energize all the tags in all reachable locations in the coverage range. While one tagged item is being scanned by one scan beam at one location in a venue, the presence, the identity, and the activity of another tagged item in other locations not covered by the one scan beam are not being observed.
More particularly, it is known to utilize an overhead RFID reader with a phased array antenna in a retail venue in which a first group of its scan beams cover an inventory location at which the tagged items are stored in order to determine, for example, a count of how many tagged items are currently at the inventory location, while a second group of its beams cover a portal, zone or exit through which the tagged items are removed from the retail venue in order to determine, for example, a count of how many tagged items have currently exited the retail venue. Both counts are useful information, and it is desired that both counts be accurate and up-to-date. However, the more time spent in scanning and counting by one of these groups, the less time is available in scanning and counting by the other of these groups. Thus, if the RFID reader is busy scanning the inventory location, then it cannot accurately scan the portal. Of course, if more time is allotted to scanning the portal, then the less time is available to accurately determine how many items are in current inventory.
Accordingly, there is a need to efficiently and dynamically adjust the scan rate of the scan beams generated by the RFID reader such that the inventory location and the portal are both adequately covered by the scan beams for sufficient time periods to yield accurate item counts and item identifications.